The present invention relates to a method and an apparatus for measuring light absorption spectra, using laser spectra.
Recently, a laser spectral technique near an infrared zone has been used for a medical diagnosis, an environmental instrumentation, a food inspection, a spectral analysis of the impurity in gas (for example, moisture in nitrogen or hydrogen chloride), a leakage detection monitor of a gas for semiconductor production, an observation of the gas concentration in a semiconductor processing apparatus and the like. The laser spectral technique is briefly described hereinafter.
The type of the atom or molecular to be measured is identified by radiating a laser beam of a certain intensity to the atom or molecular and measuring a transmittance of the laser beam at each of wavelengths of the beam.
In general, a high sensitization is one object in a laser spectral technique. In the laser spectral technique, a laser beam source is used as a light source, and the laser spectral technique has an advantage that an intensity of a measured light is extremely high when compared to a spectral technique not using a laser as a light source. Thus, it has been known as a technique providing high detection sensitivity.
However, in the conventional laser spectral technique, an interference of light can be easily occurred due to a long coherent length of the laser beam. In a measured signal obtained by a laser spectra, a number of periodic vibrations are found as an interference noise which causes an interference of light.
Such noise is resulted from the presence of weak reflection lights from the surface of various types of optical parts, for example, lens, reflector, light isolator, polarizer and the like used in the measurement system.
Accordingly, the conventional laser spectral technique has a problem that weak absorption spectra of molecular in the measurement cell cannot be identified due to an interference noise; thus, it is difficult to measure the spectra correctly. For example, the theoretical detection lower limit on an unit cell length of laser spectral for moisture in nitrogen is calculated as 5 ppb/m. However, the real, practical detection sensitivity is limited to 50 ppb/m.
As described above, since an absorption signal of impurities cannot be separated from noise signals by the influence of light interference noises in the conventional laser spectral system, a high sensitivity of the laser spectra can not be sufficiently obtained.
An object of the present invention is to eliminate the above-mentioned problem of the prior art, and to provide a method and an apparatus for measuring light absorption spectra which can remove a noise included in a detected signal and achieve a high sensitivity of a laser spectra in a good S/N ratio.
The invention provides a method for measuring a light absorption spectra characterized by comprising the steps of (i) conducting a Fourier transform of an absorption spectra measured by using a laser beam source; (ii) disassembling in a Fourier space the Fourier transformed signal into a signal resulted from a periodic vibration component and a signal resulted from a molecular absorption spectra; (iii) removing the Fourier component resulted from said periodic vibration component in a Fourier space; and (iv) conducting an inverse Fourier transform on the signal from which said periodic vibration component is removed, whereby reproducing an absorption spectra in a real space.
Further, the invention provides a method for measuring a light absorption spectra characterized by the step of conducting Fourier transform comprising (i) describing said absorption spectra measured as an even function and an odd function for which a peak position of said absorption spectra forms a symmetrical axis; and (ii) conducting Fourier transform on the even function and the odd function.
Further, the invention provides a method of measuring a light absorption spectra characterized by comprising the step of measuring quantitatively a concentration based on a peak intensity of the absorption spectra in said reproduced real space.
Further, the invention provides an apparatus characterized by using one of the above methods for measuring a light absorption spectra providing an absorption spectra from which Fourier components resulted from periodic vibration components are removed.